1. Field of the Invention
This invention relates to image processing and to a contrast enhancement apparatus using a histogram equalization method to enhance image contrast.
2. Description of the Related Art
Some known image processing systems use a histogram equalization method to expand the dynamic range of an image and increase the image contrast. Here, the dynamic range of an image is the ratio of the luminance or brightness of the brightest pixel forming the image to that of the darkest pixel. Image contrast generally increases with the dynamic range. Pages 241 and 242 of "Fundamentals of Digital Image Processing" by A. K. Jain, (Prentice-Hall, 1989) describe a global histogram equalization method that expands the dynamic range of an image. The global histogram equalization method improves contrast in areas of the image where pixels have brightness values that occur frequently throughout the image. However, global histogram equalization degrades contrast in areas of the image where pixels have brightness values that are rare in the original image. Degradation occurs because histogram equalization transforms rare brightness values to relatively common brightness values. Therefore, when pixels having rare brightness values convey important information, the information can be lost or degraded. Thus, conventional histogram equalization methods can, although very rarely, destroy image information.
Korean Patent Laid-open Publication No. 96-036551 discloses an non-overlapped locally adaptive histogram equalization method that prevents contrast degradation. The adaptive histogram equalization method divides an image into uniform sized blocks or windows that do not overlap. Then, the adaptive histogram equalization is performed for each block. The adaptive histogram equalization method does not degrade contrast in areas of the image where pixels have brightness values that are globally rare in the original image. Accordingly, loss of important information conveyed by the pixels having rare brightness values can be prevented. However, the adaptive histogram equalization method determines the histogram equalization values for pixels included in one block without using pixels included in other blocks. Thus, this adaptive histogram equalization method can degrade image quality due to discontinuity between adjacent blocks.